The present invention relates to a method for producing a lithium ion-conductive solid electrolyte for solid state lithium batteries.
Recently, with the development of portable equipment such as personal computers, handy phones and the like, there is a great demand for a battery as a power source therefor. Particularly, a lithium battery has been studied vigorously in various fields, because lithium is a substance having a small atomic weight and a high ionization energy, thereby to give the lithium battery a high energy density.
On the other hand, the conventional battery used for such equipment includes a liquid electrolyte, and has the problem of possible leakage of the electrolyte. In order to increase reliability of the battery by solving the above-mentioned problem and realize a compact and thin element, many attempts have been made in various fields to realize a solid state battery by including a solid electrolyte, instead of a liquid electrolyte. The lithium battery, in particular, has a high energy density and includes therein an electrolyte containing an organic inflammable solvent. This poses a problem of a risk of ignition or the like inside the battery cell in case of a fault of the battery. This is why there is a demand for a development of the solid state lithium battery including a solid electrolyte comprising a nonflammable solid material.
Known examples of the solid electrolyte for such battery are lithium halides, lithium nitrides, lithium oxysalts, their derivatives, or the like. Under the circumstances, however, these solid electrolytes have a problem that their ionic conductivity is too low or their potential window is too narrow to apply to a battery for practical use. For this reason, these solid electrolytes are at present substantially precluded from applications to the battery for practical use.
By contrast, lithium ion-conductive solid electrolytes composed of a sulfide glass, such as Li.sub.2 S--SiS.sub.2, Li.sub.2 S--P.sub.2 S.sub.5, Li.sub.2 S--B.sub.2 S.sub.3 and the like, and a sulfide glass doped with a lithium halide such as LiI or a lithium oxysalt such as Li.sub.3 PO.sub.4 and the like have an ionic conductivity of as high as 10.sup.-4 to 10.sup.-3 S/cm, and are expected to be applied to a battery.
The sulfide glasses are prepared by mixing a glass forming material such as SiS.sub.2, P.sub.2 S.sub.5, B.sub.2 S.sub.3 and the like with a glass modifier LiS.sub.2 and thermally melting the mixture, followed by quenching. Particularly, since SiS.sub.2 included in a solid electrolyte of Li.sub.2 S--SiS.sub.2 system is a sulfide having a higher boiling point than that of P.sub.2 S.sub.5 and B.sub.2 S.sub.3, thermal melting of the glass material under a closed atmosphere is unnecessary. This makes the solid electrolyte including SiS.sub.2 one of the most suitable materials for mass synthesis.
The following are the known methods of synthesizing the lithium sulfide as a starting material of the Li.sub.2 S--SiS.sub.2 system solid electrolyte:
L-1: To heat lithium sulfate in the presence of an organic substance such as sucrose or starch in an inert gas atmosphere or under reduced pressure thereby to reduce the lithium sulfate; PA1 L-2: To heat lithium sulfate in the presence of carbon black or powdered graphite in an inert gas atmosphere or under reduced pressure thereby to reduce the lithium sulfate; PA1 L-3: To thermally decompose a lithium hydrogensulfide-ethanol in a hydrogen stream; and PA1 L-4: To heat metal lithium in the presence of hydrogen sulfide or sulfur vapor under normal pressure or under pressure thereby to cause their direct reaction. PA1 S-1: To react silicon oxide with aluminum sulfide in an inert gas atmosphere; PA1 S-2: To thermally decompose an organic compound of silicon; and PA1 S-3: To react hydrogen sulfide and silicon in a hydrogen gas atmosphere.
The following are the known methods of synthesizing the silicon sulfide as another starting material of the Li.sub.2 S--SiS.sub.2 system solid electrolyte:
However, the Li.sub.2 S--SiS.sub.2 system solid electrolyte prepared from the starting materials of the lithium sulfide and the silicon sulfide obtained by the above-mentioned conventional methods of synthesis has the following drawbacks:
First, in the methods of L-1 and L-2, an organic substance such as sucrose or starch or a carbon material such as carbon black or powdered graphite is added in excess in order to cause complete reduction of the lithium sulfate. As a result, carbon resulting from thermal decomposition of the organic substance or carbon material added is liable to remain in the resultant lithium sulfide. Besides, while lithium sulfate is hydrophilic, carbon black or powdered graphite, and carbon resulting from the thermal decomposition of the organic substance are lipophilic. This renders it difficult to disperse both substances homogeneously, resulting in unsatisfactory reduction of the lithium sulfate. Furthermore, a large amount of carbon is likely to remain in the resultant lithium sulfide. Therefore, if the lithium sulfide synthesized by the above-mentioned methods of L-1 and L-2 is used as the starting material for the solid electrolyte, carbon remains in the resultant electrolyte, and the solid electrolyte has undesirable electronic conduction which should not exist in it.
In the method of L-3, since preparation of the starting material lithium hydrogensulfide-ethanol is complicated and costs so much, this material per se does not seem to be applicable as a material for the battery.
The method of L-4 where metal lithium is directly reacted with hydrogen sulfide or vaporized sulfur has a problem that since the reaction is explosive and proceeds at high temperature, it must be performed at low temperature. As a result, the reaction proceeds only on a surface or in the vicinity of metallic lithium, and the metallic lithium tends to remain in the synthesized lithium sulfide. If such lithium sulfide is used as the starting material of the solid electrolyte, the metallic lithium remains in the resultant electrolyte, and undesirable electronic conduction is prone to occur in the electrolyte. Furthermore, if such electrolyte is included in the solid state lithium battery, an electrode active material is reduced by the residual metallic lithium, rendering it difficult to obtain desired battery characteristics.
Next, silicon sulfides synthesized by the methods of S-1 to S-3 will be described.
First, if the solid electrolyte is prepared from the silicon sulfide synthesized by the method of S-1, the manufacturing cost becomes high because the cost of aluminum sulfide is relatively high. Moreover, aluminum oxide remains in the resultant solid electrolyte. Since aluminum oxide is an electrically insulating substance, the resultant electrolyte has low ionic conductivity.
Similarly, the use of the silicon sulfide synthesized by the method of S-2 for the solid electrolyte has a similar problem of high manufacturing cost of the solid electrolyte, because the starting material, organic silicon compound, is costly.
The method of S-3 has a problem that since the synthesis of silicon sulfide is performed at a high temperature of not lower than 1200.degree. C., the reaction chamber is damaged, facilitating migration of components of the chamber into the synthesized silicon sulfide. If the silicon sulfide synthesized by this method is included in the solid electrolyte, the chamber components are contained as an impurity in the resultant solid electrolyte. As a result, oxidation-reduction of the impurity may occur, thereby to cause adverse effects on the electrochemical characteristics of the solid electrolyte, such as decreased decomposition voltage.